The exemplary embodiment relates to printing systems and to methods of printing. It finds particular application in conjunction with a system for reducing non-uniformity of printed images, and will be described with particular reference thereto.
Image non-uniformity occurs in the output images of digital imaging devices, such as copiers, scanners, and printers, for a variety of reasons. Even relatively small non-uniformities can give rise to visibly objectionable print defects. In printing systems, physical alignments, component tolerances, wear and component age can influence the uniformity with which colorants, such as inks and toners, are laid down across the surface of print media. Streaks, for example, are one-dimensional image defects that generally run parallel to the process direction in the printed image. They can arise from non-uniform responses of the subsystems of a marking engine and can be constant over time in that they appear in relatively the same location from print to print. Photoreceptor scratches, contamination of the charger wire, non-uniform LED imager output and Raster Output Scanner (ROS) spot size variations, and spatially varying pressure on a bias transfer roll are examples of subsystem defects which can give rise to rendered image streaking in a xerographic marking engine. Bands are also one-dimensional image defects that generally run perpendicular to the process direction in a printed image. They are typically caused by time-varying performance of a marking engine subsystem, such as non-uniform velocity of the photoreceptor drive, out-of-roundness of development rolls, and wobble of the ROS polygon mirror. In a uniform patch of gray, streaks and bands may appear as a variation in the gray level. In general, “gray” refers to the optical density or area coverage value of any single color separation layer, whether the colorant is black, cyan, magenta, yellow, or some other color. Other defects which may arise include and mottle and graininess. Both of these are two dimensional variations in gray level, which take the appearance of dots or small irregular shapes. Graininess is similar to mottle but the variations are smaller in size.
Imaging devices generally include a processing component which converts color input values for an image into corresponding output values for each of the colorants to be used in rendering the image. For color images, bitmaps, each forming a color separation, are combined. Each color separation may be defined by a number of gray levels. The multiple color separations are combined together at printing to yield the final color print. Commonly, color documents are formed using cyan, magenta, and yellow colorants or cyan, magenta, yellow, and black colorants. A larger number or alternative colorants may also be used.
In half-tone printing, each color separation is represented by dots. Each dot can be made up of a number of pixels or elements, usually arranged in a grid, which are binary in the sense they can be either fully on or fully off. Part of the processing of input digital images, in which pixels may be defined on a continuous tone image, includes determining which pixels of each dot of the image should be on and which off. Typically, a mask is used which specifies for each gray level, which pixels of the dot are on and which are off. As more pixels are turned on, the size of the dot increases and its apparent gray level to the eye increases.
One-dimensional Tone Reproduction Curves (TRCs) are widely used in digital imaging as a means for compensating for non-linearities introduced by an individual imaging device. Some success has been achieved in the spatial uniformity correction of monochrome images for banding and streaking by appropriate modification to the tone reproduction curve. In the case of streaking, for example, the TRC is modified as a function of position in the cross-process direction. However, the computational cost of such correction methods is high, particularly in the case of color images.